1. Field of the Invention
The present invention relates to an improvement on a focus detecting device for use in a digital still camera or the like.
2. Description of Related Art
In a digital color camera, a solid-state image sensor, such as a CCD or a CMOS sensor, is exposed to an image of an object for a desired period of time in response to the depression of a release button. Then, the thus-obtained image signal, which represents a still image for one picture, is converted into a digital signal, and is then subjected to a predetermined process, such as a YC process, so that an image signal of the predetermined form is obtained. The digital image signal, which represents the picked-up image, is recorded in a semiconductor memory for every image. The thus-recorded image signal is read out, as required, to be reproduced into a signal adapted for being displayed or printed, and is outputted to a monitor or the like for displaying an image.
In such a digital still camera, heretofore, focus detection for a photographic lens is performed by using an image sensor which is provided for picking up an object image. For that purpose, in many cases, a focus detecting device of the contrast detection method is used. In the focus detection of the contrast detection method, the sharpness of an object image formed by an image pickup optical system is obtained by evaluating the output of the solid-state image sensor with a predetermined function, and the position of the photographic lens on the optical axis thereof is adjusted in such a way as to make the thus-obtained functional value become a maximum. As the evaluation functions employed in the above focus detection, there are a method of adding together absolute values of differences between respective adjacent luminance signals within a focus detecting area, a method of adding together squares of differences between respective adjacent luminance signals within a focus detecting area, a method of adding together differences between respective adjacent signals in each of R, G and B image signals, etc.
In addition, there is known, as disclosed in U.S. Pat. No. 4,410,804, a focus detecting device of the phase-difference detection method. In the focus detection of the phase-difference detection method, a pair of or two pairs of light-receiving parts are provided for respective micro lens arrays which are arranged in two-dimensional manner, and every light-receiving part is projected onto the pupil of an image pickup optical system by the corresponding micro lens array. Then, object images are formed by using two light fluxes having passed through different portions of the pupil of the image pickup optical system, and the positionally phase difference between the two object images is detected on the basis of the output of an image sensor and is then converted into the amount of defocus of the image pickup optical system.
In the focus detection of the phase-difference detection method, it is possible to obtain not only the direction of defocus but also the amount of defocus itself. Accordingly, there is an advantage that it is possible to greatly reduce a period of time required for realizing an in-focus state, as compared with the contrast detection method.
Further, there is known, as disclosed in Japanese Laid-Open Patent Application No. Sho 59-146010, a focus detecting device having both of the contrast detection method and the phase-difference detection method.
Incidentally, each of the phase-difference detection method and the contrast detection method is arranged to act by capturing the luminance distribution of an object and is, therefore, called the passive-type focus detection, which is distinguished from the active-type focus detection in which focus detection is performed by emitting rays of light toward the object from the camera.
In the case of the passive-type focus detection, the success or failure in focus detection depends on the luminance distribution of an object serving as a focus detection target. The luminance distribution exists in various ways, including a way in which the luminance is distributed in a random manner, a way in which the luminance is distributed only in the vertical direction, a way in which the luminance is distributed only in the horizontal direction, etc. For example, in a case where a person wearing vertical-striped clothes is to be photographed, if a focus detecting point is adjusted to the clothes, the luminance of the person almost never changes in the vertical direction, which is parallel with the stripe of the clothes, and greatly changes in the horizontal direction, which is perpendicular to the stripe. It goes without saying that, if there is no change of the luminance of the object, an effective output can not be obtained in the passive-type focus detection. Accordingly, it is understood that the direction of the luminance distribution giving sensitivity is an extremely important factor for the passive-type focus detecting device.
In the above-mentioned U.S. Pat. No. 4,410,804, there is also disclosed a focus detecting device of the phase-difference detection method having sensitivity both in the luminance distributions in the vertical direction and horizontal directions. Thus, the focus detection giving sensitivity both in the luminance distributions in the vertical direction and horizontal directions is realized by providing two pairs of, namely, four, photoelectric transducing parts for every micro lens. However, the provision of four photoelectric transducing parts for every micro lens necessitates the extremely-advanced minimization of a solid-state image sensor, so that it is difficult to supply such a solid-state image sensor, as an image sensor for digital cameras, at a low price.
In accordance with an aspect of the invention, there is provided a focus detecting device, comprising a sensor part arranged to receive a light flux passing through an image forming lens, the sensor part including a first pixel part having pixels arrayed in a first direction and a second pixel part having pixels arrayed in a second direction different from the first direction, and a computing circuit arranged to obtain information for focusing by performing a phase-difference computing operation for computing a phase difference between two images formed with light fluxes passing through different exit pupils of the image forming lens on the basis of a signal obtained by the first pixel part and by performing a computing operation different from the phase-difference computing operation on the basis of a signal obtained by the second pixel part.
In accordance with another aspect of the invention, there is provided a focus detecting device, comprising a sensor composed of a plurality of pixel units, each pixel unit having a first photoelectric transducing element and a second photoelectric transducing element, an optical element arranged to cause, among light fluxes passing through first and second exit pupils of an image forming lens, a light flux passing through the first exit pupil to be received by the first photoelectric transducing element of each pixel unit and to cause a light flux passing through the second exit pupil to be received by the second photoelectric transducing element, and a computing circuit arranged to compute a focusing state on the basis of a phase difference between a signal obtained by the first photoelectric transducing element of each pixel unit and a signal obtained by the second photoelectric transducing element of each pixel unit and arranged to compute a focusing state by using, as a signal for each pixel unit, a signal obtained by combining signals obtained by the first photoelectric transducing element and the second photoelectric transducing element of each pixel unit.
In accordance with a further aspect of the invention, there is provided a focus detecting device, comprising a focus detecting control part having a first mode, for small defocus, of causing a member having two aperture parts to be located between an image forming lens and a sensor part and causing a light flux passing through the two aperture parts to be received by the sensor part, and a second mode, for large defocus, of causing a light flux passing through the image forming lens without passing through the two aperture parts to be received by the sensor part, and a computing circuit arranged to detect a phase difference between two images received by the sensor part irrespective of the first mode or the second mode.
In accordance with a still further aspect of the invention, there is provided a focus detecting device, comprising a sensor part composed of a plurality of pixel units, the plurality of pixels units including at least a first array in which a plurality of pixel units are arrayed in a first direction and a second array disposed parallel with the first array, each pixel unit having a first photoelectric transducing element and a second photoelectric transducing element, the plurality of pixel units constituting the first array being disposed in such a way as to receive a light flux passing through a first color filter at intervals of a predetermined number of pixel units, the plurality of pixel units constituting the second array being disposed in such a way as to receive a light flux passing through the first color filter at intervals of a predetermined number of pixel units, every pixel unit of the second array arranged to receive a light flux passing through the first color filter being shifted by a predetermined number of pixel units from every pixel unit of the first array arranged to receive a light flux passing through the first color filter, an optical element arranged to cause, among light fluxes passing through first and second exit pupils of an image forming lens, a light flux passing through the first exit pupil to be received by the first photoelectric transducing element of each pixel unit and to cause a light flux passing through the second exit pupil to be received by the second photoelectric transducing element of each pixel unit, and a computing circuit arranged to obtain a phase difference on the basis of information on a phase difference between signals obtained by the first photoelectric transducing element and the second photoelectric transducing element of every pixel unit of the first array arranged to receive a light flux passing through the first color filter and information on a phase difference between signals obtained by the first photoelectric transducing element and the second photoelectric transducing element of every pixel unit of the second array arranged to receive a light flux passing through the first color filter.
The above and further aspects and features of the invention will become apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.